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Electrical Theory & Applications for HVACR
Chapter 3: Motors
Page 71
InsulaƟon Class (INSL):
A designaƟon for the type of insulaƟon used; primarily used for rewinding
purposes
Service Factor (S.F.):
The amount of overload that a motor can tolerate on a conƟnuous basis at
rated voltage and frequency.
Example:
A motor rated at 0.75 Hp with a service factor of 1.2 is rated at 0.9 Hp.
(0.75 HP X 1.2 SF = 0.9Hp)
VARIABLE FREQUENCY DRIVES (VFDs)
Variable frequency drives are used to vary the speed, torque, horsepower, and direcƟon of single and
three‐phase AC motors. The input AC voltage passes through a filtered recƟficaƟon circuit to change
it to direct current. By using electronic switches, the frequency can be varied from approximately 1
to 400 hertz. Today's VFD’s have programmable opƟons which can be programmed changing for
different applicaƟons. When changing the voltage and the frequency at the same Ɵme, a motor will
produce approximately 150% of torque with only 50% of current. The controller can be programmed
to slowly ramp the motor up to speed and gradually slow it down before stopping. This eliminates
the high mechanical stress developed from starƟng and stopping. When an AC motor needs to be
prevented from freewheeling or needs to come to a stop very quickly, a low DC current can be
applied to the windings, which will act as a break. Infinite capacity control may be obtained when
used in conjuncƟon with air condiƟoning or refrigeraƟon compressors. For other applicaƟons,
minimum and or maximum speeds can be set.
VARIABLE-SPEED DRIVES (VSDs)
Variable speed drives can be used on DC (direct current), single‐phase and three‐phase AC motors.
They differ from variable frequency drives in switching acƟon. The principle mode of operaƟon is
pulse with modulaƟon. The amount of Ɵme the switches are opened and closed is used to vary
motor speed. The controls can range in voltages from 110 to 10Kv.
Fig. 3‐38: Simple VFD diagram